Most of our lymphocytes are not sitting in our lymphoid organs but lining mucosal surfaces. It makes sense that this should be so, as most infections come in either through the gut, lung or genital tract. T cells expressing the gd T cell receptor have a particular association with mucosal epithelial surfaces, but what they do is still largely enigmatic [1]. Although a major component of the T cell population in cattle and sheep, gd T cells are relatively scarce in the circulation and peripheral lymphoid tissues of most mammals. During ontology, gd T cells appear before ab T cells and are the first to colonize the peripheral tissues of the lung, intestine and skin [2]. Whereas ab T cells recognize peptide fragments of antigen presented by MHC proteins, many systemic human gd T cells recognize small non-peptidic molecules, and others appear to recognize non-classical MHC (e.g. CD1) [3]. They seem to have a protective role against intracellular microbial infections, particularly mycobacteria and protozoa. More controversially, they may also protect against cutaneous carcinomas [4] and perhaps suppress ab T cell-mediated inflammation in allergic states [5].In this issue of Clinical and Experimental Immunology , Aoyagi et al . report a study of 15 infants with severe lung infections due to respiratory syncytial virus (RSV) and seven with reovirus infections of the gut. They found that mitogen-stimulated gd T cells from the peripheral blood of infants with acute RSV infection produced significantly less IFN-g and more IL-4 than gd T cells from infants with acute reovirus infection. During convalescence, the percentage of gd T cells producing IFN-g increased in seven children who recovered fully but failed to do so in eight who developed postbronchiolitic wheezing. There was no significant difference in the percentage that produced IL-4, but the numbers were small. They conclude that cytokine production by gd T cells during acute RSV infection may play an important role in the development of recurrent wheeze after RSV infection.These are novel, intriguing and rather surprising results. These results suggest an important role for gd T cells not only in the pathogenesis of acute RSV disease in human infants, but also in the development of recurrent wheezing after bronchiolitis. Clearly, different types of wheezing disorder can arise by distinct immunopathogenic mechanisms. What is true of post viral wheeze in young children is not at all a guide to what happens in atopic asthma in older children and young adults, and both are certain to be due to different pathogenic mechanisms from nonatopic 'intrinsic' asthma of later onset. However, many of the patients in the current study were from atopic families and it is important to recognize that this study involved very few patients. Moreover, the authors used mitogen rather than specific antigens to drive cytokine production and only studied IL-4 and IFN-g . The convalescent responses were studied in the RSV group but not in the